liblock: make lockdep_set_thread an exported function

This will let us split the public and private headers further.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
liblock: completely split public and private headers

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
liblock: remove unneeded extern from tests

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>